One conventional method of manufacturing a multi-colored design on a plastic substrate is hot stamping, in which a roll leaf is stamped with heated metal dies onto a plastic face, thus engraving the desired marking or design into the plastic. Hot stamping to form a multi-colored image suffers from a significant disadvantage in that it is considerably expensive since it requires a costly metal or silicone rubber printing plate. Furthermore, such printing components are rather complex and tedious to operate and maintain. In addition, depending on the surface of the object to be stamped, it may be difficult to produce an image with a sharp profile or contrast.
Another common method of producing a multiple colored design on a plastic substrate is the pad printing technique. This method, as generally disclosed in U.S. Pat. No. 5,513,567, entails first applying a base coat material over a substrate surface, then printing a desired image from an ink that adheres to the base coat material, and finally applying an adhesive top coat material over the ink image. This method suffers from a disadvantage of being burdensome and costly in manufacturing since multiple ink applications are necessary. In addition, solvent used as a diluent is an environmental hazard. Furthermore, the topcoat and/or image layer(s) can wear off before the end of the useful life of the substrate.
Laser ablation for marking plastics is another conventional method of producing a multi-color design on a plastic substrate. Traditionally, a polymer of a desired undercoat color is injected into a mold and then allowed to cure to form a first layer. Then, a differently colored polymer is injected over the cured first layer to form a second layer, and that second layer is allowed to cure. The outermost second layer is then ablated to expose the first layer with the desired color. Alternatively, two polymeric layers may be separately formed and then welded together by an adhesive layer or cured together to form a multi-layered polymer substrate. The top layer or outer layer of the substrate is then laser ablated in order to expose the underlying colored layers in the shape of the desired pattern. This method suffers from a significant disadvantage in that multiple steps of welding or adhesive bonding two or more polymeric layers together are necessary. And, in the previously noted related version of this technique, multiple injection molding and multiple curing phases are required.
Multiple material molding (sometimes referred to as "multi-material molding") techniques produce components having graphics through the use of highly specialized molding machines that utilize multiple injection barrels. Multiple material molding requires one injection barrel (sometimes referred to as a screw assembly) for each material that is utilized. These barrels inject colored material into a mold, wherein the colored materials are directly formed into a component having a graphic and a background. Multiple material molding techniques require substantial investment in tooling costs, particularly since an injection barrel is required for each color that is to be utilized. The simplest arrangement for a component having a graphic requires the use of materials of two colors: a first color for the graphic and a second color for the background. Such an arrangement requires two injection barrels, one barrel for each color. For many applications, the requirement of multiple barrels results in prohibitively high tooling costs. In addition to cost considerations, the color appearance of multiple-color graphics produced by prior multi-material molding methods is often unsatisfactory.
"Paint-and-laser" techniques involve producing a transparent plastic substrate that is painted white or another light color to form a white translucent layer over the transparent substrate. The component produced thereby is then painted a relatively dark color, such as black, to form an opaque covering over the white translucent layer. The black covering is then lased away to expose a portion of the white translucent material. The exposed portion of translucent material forms the graphic. Paint-and-laser techniques are described, for example, in U.S. Pat. No. 5,432,684 to Fye et al. assigned to the same assignee as the present application.
Paint-and-laser techniques are typically utilized in the production of components having a single color graphic (for example, a white graphic and a black background), since the technique involves sequential painting of the component with one color followed by another color. However, if the graphic requires multiple colors, expensive and time-consuming paint masking operations must generally be incorporated into the technique.
There are additional disadvantages associated with these prior art techniques. Paint-and-laser multicolor techniques also have significant cost and/or quality limitations. The previously noted hot stamp technique has historically resulted in high numbers of unusable ("high scrap") products and often suffers from lifted edges and undesirable witness lines. Witness lines are readily apparent topographies that are not hidden by layers of paint. In a foil or heat transfer operation, the thickness of the heat transfer or printed block of ink underneath a dark background color will often be visible in the final product. The pad printing color technique previously mentioned also suffers from witness lines due to the required thickness of the ink. Witness lines can in some cases be masked, at least partially, by using carefully chosen patterns and multiple printings. However, pad printing, for example, generally suffers from the inability to produce different colors close to each other and results in an expensive product due to the number and complexity of processing steps and the typically high scrap rates.
Sublimation printing has also been used to produce multiple color graphics in certain products. In this technique, solid inks disposed on a carrier are brought into contact with an area on a substrate that is to be colored. Through the application of pressure and heat, dye or colorant in the ink is driven into the substrate. Under certain process conditions, the dyes change state from a solid to a gas (i.e., sublime), thereby facilitating penetration into the surface of the substrate. The sublimation process has been used to deposit dark graphics on opaque, light-colored computer keyboard key caps. Sublimation printing has also been used to form colored designs on white or light-colored plastics and synthetic fabrics. Sublimation dyes are available from a variety of suppliers as either screen printing inks or as inks pre-printed on a carrier in custom designs. However, sublimation printing suffers from a disadvantage in that it is more difficult, if not impossible, to obtain a light image on a dark background.
A common problem of all of the traditional methods is that forming specific hues and shades of a desired color in a multi-colored image is often very difficult. For example, when hot stamping, the desired color, through the process of heating and stamping, may lose its color, fade, or otherwise become discolored. It is also difficult to use any of the processes to form the desired shade or hue of a light colored image upon a dark background.
For at least the foregoing reasons, there is a need for a technique for producing a multi-colored image or marking on a plastic substrate that does not suffer from the disadvantages listed above.